1. Field of the Invention
The present invention relates to a minute switching device manufactured using MEMS technology, and a method of manufacturing such a switching device.
2. Description of the Related Art
In the technical field of wireless communication equipment such as mobile phones, as for example the number of components installed in the equipment is increased to realize improved performance, there have been increased demands to miniaturize high-frequency circuitry and RF circuitry. To answer to these demands, there have been advances in miniaturization using MEMS (micro-electromechanical systems) technology for various components constituting the circuitry.
A MEMS switch is an example of such components. Specifically, a MEMS switch is a switching device in which each part is formed minutely using MEMS technology. The switch may include a pair of contacts for carrying out switching by mechanically opening/closing, and a driving mechanism for achieving the mechanical opening/closing operation of the contacts. In switching of high-frequency signals of GHz order in particular, a MEMS switch can exhibit higher insulation in the open state and lower insertion loss in the closed state than a switching device incorporating a PIN diode, a MESFET or the like. This is due to the open state being achieved through mechanical opening between a pair of contacts, and the parasitic capacitance being low due to being a mechanical switch. MEMS switches are disclosed in Japanese Patent Application Laid-open No. 9-17300 and Japanese Patent Application Laid-open No. 2001-143595, for example.
FIGS. 24 and 25 show a conventional MEMS micro-switching device X4. The micro-switching device X4 includes a substrate 401, a movable portion 402, a movable contact part 403, a pair of stationary contact electrodes 404, and driving electrodes 405 and 406. The movable portion 402 has an anchor portion 402a that is joined to the substrate 401, and an arm portion 402b extending out from the anchor portion 402a along the substrate 401. The movable contact part 403 is provided on a lower surface of the arm portion 402b. The driving electrode 405 is provided on an upper surface side of the arm portion 402b. A wiring part 407, continuing on from the driving electrode 405, is provided on the movable portion 402. The pair of stationary contact electrodes 404 are disposed on the substrate 401 in a manner such that one end of each of the stationary contact electrodes 404 faces the movable contact part 403. The driving electrode 406 is grounded and provided on the substrate 401 in a position corresponding to the driving electrode 405. Prescribed wiring patterns (omitted from the drawings) electrically connected to the stationary contact electrodes 404 and the driving electrode 406 are formed on the substrate 401.
With the micro-switching device X4 having the above arrangement, when a prescribed potential is applied to the driving electrode 405 via the wiring part 407, an electrostatic attractive force is generated between the driving electrodes 405 and 406. As a result, the arm part 402b elastically deforms to a position in which the movable contact part 403 contacts the stationary contact electrodes 404. In this way, the closed state of the micro-switching device X4 is achieved. In the closed state, the stationary contact electrodes 404 are electrically bridged by the movable contact part 403, and hence a current is allowed to pass between the stationary contact electrodes 404.
When the electrostatic attractive force acting between the driving electrodes 405 and 406 is eliminated, then the arm part 402b returns to its natural state, and hence the movable contact part 403 separates away from the stationary contact electrodes 404. In this way, the open state of the micro-switching device X4 as shown in FIG. 25 is achieved. In the open state, the stationary contact electrodes 404 are electrically isolated from one another, and hence a current is prevented from passing between the stationary contact electrodes 404.
FIGS. 26A-26D and 27A-27D show some of the steps in a method of manufacturing the micro-switching device X4. In the manufacture of the micro-switching device X4, first, as shown in FIG. 26A, the stationary contact electrodes 404 and the driving electrode 406 are pattern-formed onto the substrate 401. Specifically, a film of a prescribed electrically conductive material is formed on the substrate 401, and then a prescribed resist pattern is formed on the electrically conductive film using a photolithography method, and the electrically conductive film is subjected to etching treatment using the resist pattern as a mask. Next, as shown in FIG. 26B, a sacrificial layer 410 is formed. Specifically, using for example a sputtering method, a prescribed material is deposited or grown on the substrate 401 so as to cover the stationary contact electrodes 404 and the driving electrode 406. Next, through etching treatment carried out using a prescribed mask, as shown in FIG. 26C, a single recess 411 is formed in the sacrificial layer 410 in a place in correspondence with the stationary contact electrodes 404. Next, as shown in FIG. 26D, a film of a prescribed material is formed in the recess 411, thus forming the movable contact part 403.
Next, as shown in FIG. 27A, a material film 412 is formed using, for example, a sputtering method. Next, as shown in FIG. 27B, the driving electrode 405 and the wiring part 407 are pattern-formed on the material film 412. Specifically, a film of a prescribed electrically conductive material is formed on the material film 412, and then a prescribed resist pattern is formed on the electrically conductive film using a photolithography method, and the electrically conductive film is subjected to etching treatment using the resist pattern as a mask. Next, as shown in FIG. 27C, the material film 412 is patterned, thus forming a film body 413 constituting the arm part 402b and part of the anchor part 402a. Specifically, a prescribed resist pattern is formed on the material film 412 using a photolithography method, and then the material film 412 is subjected to etching treatment using the resist pattern as a mask. Next, as shown in FIG. 27D, the other part of the anchor part 402a is formed. Specifically, the sacrificial layer 410 is subjected to isotropic etching treatment via the film body 413 which acts as an etching mask, this being such that an undercut is formed below the arm part 402b while the abovementioned other part of the anchor part 402a is formed by being left behind.
One of the properties required of a switching device is low insertion loss in the closed state. Moreover, given that a reduction in the insertion loss of the switching device is to be aimed for, it is desirable for the electrical resistance of the stationary contact electrodes to be low.
However, with the micro-switching device X4 described above, it is difficult to make the stationary contact electrodes 404 thick, and in actual practice the thickness of the stationary contact electrodes 404 is about 2 μm at most. This is because it is necessary to secure the flatness of the upper surface in the drawing (the growth end face) of the sacrificial layer 410 that is temporarily formed in the process of manufacturing the micro-switching device X4.
As described above with reference to FIG. 26B, the sacrificial layer 410 is formed by a prescribed material being deposited or growing on the substrate 401 so as to cover the stationary contact electrodes 404. The growth end face of the sacrificial layer 410 will thus become stepped due to the thickness of the stationary contact electrodes 404. The thicker the stationary contact electrodes 404, the larger the steps, and the larger the steps, the more difficult it tends to become to form the movable contact part 403 in the proper position or form the arm part 402b in the proper shape. Moreover, in the case that the thickness of the stationary contact electrodes 404 is greater than a certain value, the sacrificial layer 410 formed on the substrate 401 may break due to the thickness of the stationary contact electrodes 404. If the sacrificial layer 410 breaks, then it will not be possible to form the movable contact part 403 and the arm part 402b on the sacrificial layer 410 properly. With the micro-switching device X4, it is thus necessary to make the stationary contact electrodes 404 sufficiently thin that inappropriate steps are not formed on the growth end face of the sacrificial layer 410. With the micro-switching device X4, it may thus be difficult to realize a sufficiently low resistance for the stationary contact electrodes 404, and as a result it may not be possible to realize a low insertion loss.